The present invention relates to a method and apparatus to provide an acoustic source, and more particularly to a shaftless acoustic source suitable for sensing mechanical properties of a formation.
An effective acoustic source is a key component for any sonic imaging tool, particularly for deep acoustic imaging surveys. To function effectively during deep image surveys, the acoustic source should radiate a wideband pulse efficiently so as to generate compact time domain signatures. These pulses ideally should have energetic low frequency components to propagate significant distances into the formation. At the same tine, these pulses should generate a minimum of Stoneley and tool modes.
Traditionally, enhancement of the low frequency range of the spectrum has been problematic for two reasons. First, borehole dimensions limit the possibility of building low frequency resonators to enhance this range of the spectrum. Second, low frequency sources tend to excite the less preferred Stoneley mode. This latter effect is especially true for traditional fluid coupled monopole sources where the impedance mismatch between borehole fluid and the formation allows only a small percentage of the energy to be radiated. Most of the energy propagates within the borehole as the less preferred Stoneley mode. As a result of these effects, previous attempts to build compact, simple and efficient low frequency borehole sources have been only marginally successful.
Because it is difficult to isolate the energy source from the body of the tool, many traditional source designs have the additional tendency to generate tool borne arrivals in the same frequency band as the signal. Accordingly, the ability to generate tool arrivals that are much smaller in relative magnitude and in a different frequency band from the signal is a desirable characteristic of any new source design.
There are numerous methods and apparatuses that have been used to isolate the energy source from the body of the tool or to attenuate desired frequencies. These designs have not been entirely successful in addressing the issues discussed above. For example, commonly owned U.S. Pat. No. 5,387,767, entitled xe2x80x9cTransmitter for Sonic Logging-While-Drillingxe2x80x9d to Aron et al., discloses a piezoelectric source that is isolated from a drilling collar using various springs. Likewise, commonly owned U.S. Pat. No. 5,753,812, entitled xe2x80x9cTransducer for Sonic Loggingxe2x80x9d to Aron et al., discloses an isolated piezo-electric ceramic transmitter mounted within an elastomer body seated in a groove of a drilling collar. In addition, U.S. Pat. No. 6,188,961, entitled xe2x80x9cAcoustic Logging Apparatus and Methodxe2x80x9d to Mandal, discloses a single dipole source and multiple receiver apparatus that uses an isolator to prevent unwanted signals. Further, U.S. Pat. No. 6,213,250, entitled xe2x80x9cTransducer for Acoustic Loggingxe2x80x9d to Wisniewski et al., discloses a transducer housing that is removed from the axis of a tool body.
Several apparatuses and methods have been developed to dampen unwanted signals. For example, commonly owned U.S. Pat. No. 5,309,404, entitled xe2x80x9cReceiver Apparatus for Use in Logging-While-Drillingxe2x80x9d to Kostek et al., discloses a receiver/transmitter configuration designed to reduce the affect of unwanted modes. Likewise, U.S. Pat. No. 6,145,615, entitled xe2x80x9cMechanic Filterxe2x80x9d to Beresford et al., discloses a drill string design that enables acoustic isolation between a drill bit and the drill string. The drill string includes a section with a spring and a mass that acts to dampen longitudinal compression waves.
The above-referenced patents are incorporated by reference herein in their entirety.
Accordingly, there exists a need for an apparatus and a method to generate efficient low frequency, high energy pulses that propagate significant distances into a formation while minimizing the generation of Stoneley and tool modes.
It is an object of the present invention to provide an apparatus and a method for the generation of acoustic signals useful in deep imaging applications and overcoming the limitations of the prior art. More specifically, it is an object of the present invention to provide a shaftless apparatus that is simple and compact yet generates a radiated signal sufficient for deep acoustic image surveys. It is yet another object of the present invention to provide an acoustic source that produces a reaction force on the body of a drilling tool that is both weaker in relative magnitude and lower in frequency than the signal.
Many traditional acoustic energy sources are based on a hammer design in which the hammer""s head strikes a blow creating the energy source. The hammer""s shaft controls the motion of the blow and transmits the striking force to the head. Part of the kinetic energy from each blow of the hammer converts to acoustic energy during impact. However, in a borehole environment the shaft is an undesirable element because it allows significant acoustic energy leakage into borehole fluid as it reverberates after each impact. As disclosed herein, gun coil configurations (generally referred to herein as an acoustic source) are an exceptional alternative to shaft-based sources and overcome many of the limitations of the prior art.
A first embodiment of the present invention includes a shaftless acoustic source having a solenoid, a bullet and driving electronics. The bullet, which is positioned inside the solenoid""s coil, can be accelerated by a magnetic force generated when a current pulse is applied to the coil. Acoustic energies are generated upon the impact of the bullet with an object (i.e. the deceleration of the bullet upon impact). While the bullet may directly impact a borehole formation, direct contact is not necessary. The bullet may impact a relatively acoustically transparent media and thereby impart acoustic energy into the borehole formation. This impact force can be described by the Hertzian Contact Theory (also referred to herein as Hertzian Theory) as a point of normal force with a time function similar to a half period sine function. A retraction means may be used to retract the bullet back into the solenoid after firing.
A second embodiment teaches a method of generating an acoustic source by propelling a bullet toward said borehole wall using an electromagnetic force. The electromagnetic force is generated by providing a solenoid having a bullet positioned within its coil. A current pulse is applied to the solenoid, accelerating the bullet to impact the borehole wall.
The acoustic source may be adapted for connection to a borehole apparatus (such as a host tool, including a sonde or drill collar for logging while drilling applications, or a permanent installation). The acoustic energy generated upon impact may be used to log a borehole formation by using receivers strategically positioned within the borehole (such as on the borehole apparatus).
Depending on the size of the bullet and the different cap materials, a peak force of 3000 to 4500 pounds with contact times from 100 to 300 microseconds can be generated using the present invention. Further, acceleration improvements up to four times can be achieved by positioning a magnetic shield around the coil of the solenoid thereby enhancing the magnetic driving force.
Note that the use of the term xe2x80x9cbulletxe2x80x9d merely refers to a mass used for impact and is not intended to limit this element to any particular shape or construction.
Further features and applications of the present invention will become more readily apparent from the figures and detailed description that follows.